The present invention relates to the field of modulation and switching of light within optical waveguides.
Considerable interest has been shown in the transmission of large amounts of information from many sources to many destinations by means of beams of light transmitted through optical waveguides. Information is modulated upon the beams of light and the beams are thereafter switched appropriately through a network of light guides or fiber optic elements to be routed to desired destination. Devices for switching the light beams by electronic control involve waveguides surrounded by material of lower refractive index to confine the light. 2.times.2 "X" waveguide switches of the 2-mode interference type have a waveguide in the mid-region of the switch whose dimensions are large enough to propagate the two lowest-order modes within the waveguide. The waveguide formed at the intersection of the monomode input guides of the "X"-type switch utilizes means for controlling the electric field or free-carrier density within the guide to in turn cause changes in the index of refraction of various cross-sectional portions of the switch intersection, and as a result, the velocity of propagation of the even mode relative to the odd mode is differentially affected to switch the direction of the output light beam. U.S. Pat. No. 4,693,547 teaches utilizing an LED for switching control and U.S. Pat. Nos. 4,746,183, 4,787,691, and 4,728,167 issued to R. Soref and J. Lorenzo and incorporated herein by reference, teach utilizing pn junctions to inject free carriers into the waveguide at the X-switch intersection.
In proposed devices, the electro-optic controller consisted of an elongated diode on a rib optical waveguide. In some cases, the diode was forward-biased to inject minority carriers into the waveguide. In others, the diode was reverse-biased so as to deplete the waveguide of ambient carriers. Although injection and depletion diodes are effective phase modulators, each has its limitations. The speed of the injector is limited by the carrier lifetime, and the required injection currents are greater than 10 mA. The depletion diode is faster and consumes less power, but the depleter requires a large background concentration of impurities in the waveguide (3.times.10.sup.17 impurities/cm.sup.3) in order to attain large changes in carrier concentration during biasing. However, the moderately heavy doping requirement conflicts with other aspects of the device: it introduces propagation loss of about 3 dB/cm into the waveguide. In devices utilizing the present invention, the heavy doping requirement is relaxed because the modulated charge density may be determined jointly by double-injection combined with depletion. For further background with regard to these prior devices, see "Silicon Guided-Wave Optics" published by R. Soref and J. Lorenzo in "Solid State Technology"; November 1988, and the references cited therein.